JPS64670B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to combustible neutron absorbers (also referred to as combustible poisons) for nuclear reactors, and in particular to improvements in combustible absorber coatings for nuclear fuel. It is well known that nuclear fuel has various forms such as fuel plates, fuel columns and fuel pellets, and the fuel pellets are loaded end-to-end into zirconium alloy or stainless steel cladding tubes. .
Fuel pellets are uranium dioxide, thorium dioxide,
Contains fissile materials such as plutonium dioxide or mixtures thereof. Typically, a plurality of fuel rods are assembled to form a single fuel assembly, and the plurality of fuel assemblies are arranged together to constitute the core of a nuclear reactor. It is well known that the nuclear fission process involves the collapse of fissile fuel material into two or more fission products with lower mass numbers. Among these, the nuclear fission process generates neutrons, which are the basis of self-sustaining reactions. After a nuclear reactor has been operated for a period of time, the fuel assemblies containing fissile material eventually deteriorate and must be replaced. Since refurbishment operations are time consuming and costly, it is desirable to extend the life of a given fuel assembly to the maximum length of practical use. For this reason, the planned addition of small calculated amounts of parasitic neutron-capturing elements to reactor fuels will have a very beneficial effect on thermal neutron reactors. Such neutron-capturing elements are commonly referred to as "combustible absorbers" and they also degrade after a certain amount of time, so there is compensation for the concomitant reduction in fissile material. The life of a fuel assembly can be extended by initially combining a large amount of fissile material with a calculated amount of combustible absorbent material. During the initial stages of operation of such a fuel assembly, the combustible absorber absorbs excess neutrons and exchanges them for elements with lower neutron absorption cross sections. The element does not substantially affect its reactivity at the end of the life of the fuel assembly, when the availability of fissile material is low. Thus, with a fuel assembly containing carefully balanced amounts of both fuel and combustible absorbent material, extended life of the fuel assembly can be achieved with relatively constant neutron production rate and reactivity. Combustible absorbent materials that can be used include boron, gadolinium, salarium, and europium. The combustible absorbent can be either homogeneously mixed with the fuel (i.e., a dispersed absorbent) or separated into the reactor as a separate element so that the combustible absorbent is nearly identical to the fuel. It burns, or deteriorates, at a rapid rate. Therefore, the net reactivity of the core remains relatively constant throughout the useful life of the core. U.S. Pat. No. 3,427,222 discloses uranium dioxide fuel pellets coated with a mixture of uranium dioxide and a zirconium diboride burnable poison obtained by plasma spray technology (see examples in column 4). ing. This patent also discloses uranium dioxide fuel pellets coated with the burnable poison boron obtained by chemical vapor deposition. In this US patent, it is found that the deposition rate is slow at low temperatures, but the coating is not tacky at high temperatures (see examples in column 5). It is well known to coat nuclear fuel confined in an aluminum container with a layer of niobium to prevent the nuclear fuel from reacting with the container (see GB 859,206, page 1, lines 12-30). In addition, fine nuclear fuel particles such as uranium dioxide particles may be coated with a single layer or several layers of the same or different non-absorbing material containing niobium to protect the fuel from corrosion or to help retain the products of nuclear fission. It is also well known to coat with Coatings are obtained by various techniques such as deposition from steam, deposition from decomposed steam, electroplating etc. of the coating material (GB 933,500). ANHolden's AEC paper, “Dispersed Fuel Elements,” published in 1967, describes how fuel particles in a dispersed fuel are coated to reduce the interaction between the particles and the matrix. It talks about preventing nuclear fission and retaining the products of nuclear fission (page 30). Uranium dioxide coated with niobium by gas phase reduction is disclosed (page 48). Also disclosed is the coating of uranium dioxide with chromium by gas phase reduction using chromium dichloride deposited on a subbing layer of niobium (page 48). The inventors of the present invention have determined that the delamination problems associated with chemical vapor deposition of zirconium diboride onto uranium dioxide are due to a thin niobium subbing layer (approximately 3 mils thick) on the uranium dioxide.
It is known that a solution can be achieved by first depositing (by sputtering, chemical vapor deposition, etc.) zirconium diboride (~6 microns) and then chemical vapor depositing the zirconium diboride over the niobium basecoat layer. Fuel pellets coated with boron containing combustible absorbers such as elemental boron, isotopes of boron-10 (isotopes of elemental boron that have the properties of combustible absorbers), zirconium diboride, boron carbide, boron nitride, etc. The problem is that the degree of water absorption changes. For example, after production, uranium dioxide fuel pellets coated with zirconium diboride are dried in a furnace, a time-consuming process, and then loaded into fuel rods in a low-humidity glovebox environment. There must be. This is necessary because zirconium diboride, which is hygroscopic, picks up a thin layer of moisture from the air itself (moisture absorption).
Long drying process (200 min in vacuum below 1 mmHg)
Temperatures of ~600° C. for approximately 1 to 3 hours) and moisture-controlled pellet loading environments increase the time, complexity, and cost of nuclear fuel processing lines. Moisture in the nuclear fuel should be avoided because excess hydrogen in the fuel pellets, which primarily appears as water, can cause hydroxide formation in the Zircaloy cladding of the fuel rods, which can lead to cladding flaws. Therefore, the main object of the present invention is to have pellets coated with neutron-absorbing material that are difficult to absorb moisture so that they can be stored and then processed without lengthy and costly preparations. The purpose of the present invention is to provide a fuel element into which a fuel element can be inserted. For this purpose, the present invention provides a nuclear fuel body comprising a nuclear fuel base containing fissile material and a layer containing a combustible absorbing material covering at least a portion of the nuclear fuel base, in which a hydrophobic material that can be used in a nuclear reactor is added. including,
It is characterized by comprising a protective film layer directly bonded to and surrounding the layer of combustible absorbent material. The invention will become more readily apparent from the following description of preferred embodiments thereof, illustrated in the accompanying drawings. Nuclear fuel is uranium dioxide (or thorium dioxide,
plutonium dioxide or mixtures thereof)
It contains uranium in the form of pellets, each pellet being cylindrical with a diameter of approximately 8 mm and a length of approximately 12 mm. The preferred thickness of the combustible absorbent coating of zirconium diboride on the surface of the fuel pellet is approximately 8 to
16 microns (about 9-10 microns is preferred, corresponding to a loading target of approximately 0.6 mg boron-10 per cm linear length). The degree of moisture absorption depends on the technique used to deposit the zirconium diboride layer, with sputtering creating a slightly porous coating that contributes to moisture absorption, whereas chemical vapor deposition contributes to moisture absorption. I knew that there seemed to be fewer problems. As shown in FIGS. 1 and 2, a fuel rod 10 used in a nuclear reactor fuel assembly has a cladding tube 1 having end plugs 14 and 16 at the top and bottom and forming a chamber 18.
2, and a plurality of fissile fuel pellets 20 are placed in the chamber 18 in end-to-end contact, being pressed against the bottom end plug 16 by the action of a spring 22. The diameter of pellet 20 is slightly smaller than the diameter of cladding tube 12 so as to form gap 24. Spring 22 and gap 24 absorb any thermal expansion of pellet 20 during operation. The combustible body of the fuel pellet 20, i.e. the substrate 2
Preferably, 6 consists essentially of uranium dioxide, although plutonium or thorium can be used as well as other forms of uranium. The combustible absorbent layer 30 covering at least a portion of the base 26 is preferably made of elemental boron or zirconium diboride, but like other forms of boron, gadolinium, samarium, europium, etc. Can be used. Nuclear fuel pellets coated with combustible absorbent material 20
In order to make the material non-hygroscopic (hydrophobic), the combustible absorbent layer 30 is coated with a protective film layer 32 directly bonded thereto. Overcoat layer 32 preferably includes a reactor compatible hydrophobic material and has a thickness of about 2 to 6 microns. The protective film layer 32 is
The combustible absorbent layer 30 should be bonded before being exposed to air so that moisture (absorbed by the combustible absorbent material) is not trapped in the fuel pellets 20. Reactor compatibility factors to consider for such a protective layer include cost, neutron absorption cross section, compatibility with various combustible absorbers, cladding 1
2, melting point, etc. Therefore, hydrophobic substances compatible with nuclear reactors are of the following groups:
Niobium, zirconium, magnesium, aluminum, silicon, carbon, titanium, chromium,
iron, nickel, copper, yttrium, molybdenum,
It is recognized that it is a substance selected from barium and cerium. In a first preferred embodiment, elemental boron is used for the combustible absorbent layer 30, which is
Although bonded directly to substrate 26, which is uranium dioxide, overcoat layer 32 consists essentially of niobium.
In one example, uranium dioxide fuel pellets are deposited by chemical vapor deposition (CVD).
The technique was first coated with elemental boron and then coated with niobium using a vertical tube surrounding vertically stacked fuel pellets. The boron combustible absorbent material layer 30 was prepared by high temperature heating of B ₂ H ₆ and the niobium overcoat layer 32 was prepared by hydrogen reduction of niobium pentachloride (NbCl ₅ ).
These gaseous CVD precursors were introduced at the bottom of the vertical tube, and by-products were discharged from the top of the vertical tube.
The fuel pellet substrate 26 was cleaned by light sanding, repeated ultrasonic cleaning in distilled water, and vacuum drying. A thermocouple was attached to the wall of the vertical tube. The substrate 26 was heated by the upper furnace to a predetermined wall temperature as measured by a thermocouple, while the precursor gas was heated by the lower furnace to a predetermined wall temperature as measured by a thermocouple. . As a result, satisfactory coatings were obtained under various conditions as summarized in Table 1.

[Table] In the second preferred embodiment, FIGS.
As shown, zirconium diboride is used in the combustible absorbent material layer 30, which is used for CVD
It is bonded to an undercoat layer 28 made of niobium. The subbing layer 28 is a substrate 26 that is uranium dioxide.
are joined by CVD. Protective film layer 32
consists of CVD niobium. The necessity of a niobium undercoat layer, etc. when attaching zirconium diboride to uranium dioxide by chemical vapor deposition (CVD) has been described above. The thickness of the undercoat layer 28 is about 3~
Preferably it is 6 microns. Technically, it is the same as the first embodiment. Zirconium diboride
The CVD precursors were zirconium tetrachloride and boron trichloride. Gaseous zirconium chloride is
HCl and zirconium were reacted and the reaction products were carried into a hydrogen stream for conditioning. As a result, the second
Satisfactory coatings were obtained under various conditions summarized in the table.

【table】

[Table] The present invention circumferentially surrounds the fuel pellet substrate 26 with a combustible absorbent layer 30 and an overcoat layer 32 (and optionally a subbing layer 28) (i.e., covering only the circumferential walls). It is typically used for. However, in some cases it may be desirable to coat the entire fuel pellet substrate 26, including the top and bottom surfaces. It may also be advantageous to coat only a portion of the fuel pellet substrate with a layer of combustible absorbent material and then to cover the combustible absorbent layer in whole or in part with a protective film layer. Where the substrate, combustible absorbent layer, overcoat layer and subbing layer may each contain uranium dioxide, zirconium diboride and niobium, said substrate etc. may consist essentially of uranium dioxide, zirconium diboride and niobium, respectively. It is preferable to consist of.

[Brief explanation of drawings]

FIG. 1 is a longitudinal cross-sectional view of a fuel rod containing fuel pellets coated with a combustible absorbent material and having a non-hygroscopic protective film layer according to the invention; FIG.
3 is a longitudinal sectional view of the fuel rod of FIG. 1 provided with an undercoat layer, and FIG. 4 is a vertical sectional view of the fuel rod of FIG.
FIG. In the figure, 10 is a nuclear fuel body (fuel rod), 26 is a nuclear fuel base, 30 is a layer of combustible absorbent material, and 32 is a protective film layer.

Claims (1)

[Claims] 1. A nuclear fuel body comprising a nuclear fuel base containing a fissile material and a layer containing a combustible absorbent material covering at least a portion of the nuclear fuel base, which contains a hydrophobic material that can be used in a nuclear reactor, and wherein the combustible absorbent material 1. A nuclear fuel assembly comprising a protective membrane layer directly bonded to and surrounding a layer of material.